Latent Heats
The word latent refers to the "hidden" or intrinsic heat found within the molecules of solids or liquids as the energy required to keep these molecules bonded in the solid or liquid states. When a solid melts or a liquid vapourizes this energy is liberated in the form of heat. The higher the mass of a substance the higher the stored heat or latent heat. Different substances have different amounts of energy.
The latent heat of fusion (Qf -- for melting) can be calculated using the following equation:
Qf = lf x m
In the same way the latent heat of vapourization (Qv -- for a liquid turning into a gas or vapour) can be calculated using the following equation:
Qv = lv x m
The constants lf and lv are known as the latent heats of fusion and vapourization respectively.
Exercise #4:
The heat of vapourization of water is 2400 KJ/Kg. On a hot day (400C) a 70 kg man sweats so that 160 W of power are dissipated from his skin. How much sweat does he lose per hour through evapouration from his skin?
Exercise #5:
How much ice (-50C) is needed to bring down the temperature of a tub of water (50 L) from 700C to 200C. Assume the ice melts in the process. [lf ice = 335 KJ/Kg; lv water = 2260 KJ/Kg]
